Because of having high thermal conductivity and being lightweight, aluminum alloys are used in many heat exchangers which are mounted on automobiles, etc. A heat exchanger for exchanging heat by circulating fluid, e.g., water or oil therein is composed of various parts, such as a tank, tubes, and fins. Those parts are metallically joined to each other by brazing.
As aluminum alloy parts constituting the heat exchanger that are joined by brazing, brazing sheets are used. The brazing sheet comprises an aluminum alloy serving as a core and a filler material clad on one surface or both surfaces of the core alloy. Generally, an aluminum alloy having a melting temperature of 600° C. or higher is used as a core alloy of the brazing sheet, and an Al—Si based alloy having a melting temperature of 600° C. or lower is used as a cladding filler alloy. The heat exchanger can be manufactured through the steps of: forming individual parts of the heat exchanger by employing brazing sheets; combining the individual parts with one another; and heating them to temperature about 600° C. such that only filler alloys of the brazing sheets are melted to join the individual parts to one another. Since many parts constituting the heat exchanger can be joined at a time by employing the brazing sheets for the constituting parts, the brazing sheets are widely used as materials of various parts of the heat exchangers.
Main examples of brazing methods having been practiced so far are vacuum brazing and Nocolok brazing. The vacuum brazing employs a filler material made of an Al—Si—Mg based alloy. By heating parts to be joined in vacuum, Mg in the filler alloys is evaporated and, at that time, oxide films on the surfaces of the filler alloys are broken, thus enabling the brazing. However, the vacuum brazing is disadvantageous in requiring an expensive vacuum heating apparatus. On the other hand, the Nocolok brazing employs a filler material made of an Al—Si based alloy. After coating flux, the flux coated parts to be brazed are heated in an inert gas such that the flux breaks oxide films on the surfaces of the filler alloys, thus enabling the brazing. However, if the flux is coated unevenly, a brazing failure is caused. It is hence required to evenly coat the flux over regions where the brazing needs to be carried out.
Meanwhile, there are proposed brazing methods capable of carrying out the brazing through heating the parts to be joined in an inert gas without using the expensive vacuum heating apparatus and the flux. Patent Document 1 as listed below describes a method of brazing without using flux, the method comprising the steps of veiling the parts to be brazed containing Mg with a carbonaceous cover and heating the parts under the carbonaceous cover in an inert gas atmosphere. According to the method of the Patent Document 1, Mg acts to lower an oxygen concentration in the inner space of the carbonaceous cover and prevent oxidation of the parts placed under the carbonaceous cover, thereby enabling the brazing. Patent Document 2 as listed below describes a method of constructing a heat exchanger by employing a cladding alloy containing Mg as a filler material such that the brazing can be carried out without using flux. According to the method of the Patent Document 2, Mg in the filler material acts to remove oxide films on the surfaces of the filler material, thereby enabling the brazing.
According to any of these brazing methods without using flux, the brazing can be effectuated with Mg contained in the clad alloy and/or the filler alloy and acting to break oxide films. However, those brazing methods are difficult to realize a stable brazeability because the action of Mg breaking oxide films is weaker than those obtained with the vacuum brazing and the Nocolok brazing. The brazeability can be improved by lowering the oxygen concentration in a furnace. It is, however, difficult to maintain a low oxygen atmosphere on the industrial basis.